Element Geochemical Record Of Cretaceous Oceanic Red Beds: Implications For Paleoceanic Environment

The transform from Cretaceous Black Shales to Cretaceous Oceanic Red Beds (CORBs) indicates a change of paleoceanic environment. This paper presents high-resolution element geochemical data of CORBs from some typical sections, including the Chuangde and Tianba-B sections in Tibet, the Vispi Quarry section in Italy, and the ODP1049C Core from the North Atlantic, to study the change of paleoceanic environment. The following conclusions are drawn:Compared with overlying and underlying strata, the CORBs from different regions and different ages have similar contents of lithogenic elements (i.e., Ti, K, Mg, Rb) when normalized to Al2O3, suggesting a homogenous source area. There is no direct relationship between terrigenous detrital input and genesis of CORBs.Geochemical data show that the CORBs were deposited in a more oxic condition near the sediment-water interface, compared with overlying and underlying strata. This conclusion is supported by: (1) extremely low TOC in the CORBs; (2) high Fe³⁺/TFe ratio and depleted U content; (3) redox-sensitive elements like V, Cr, Co, Ni in the CORBs are depleted or slightly enriched, with ratios of Ni/Co<5, V/Cr<2; and (4)negative Ce anomaly.Paleoproductivity proxy and Ca contents indicate that there are some differences among CORBs from different regions and different ages.The red shale of the 32th layer from the Chuangde section reflects a high productivity and a strong oxic bottom water condition and the depositional interface occurred in the vicinity of the CCD. The 34th layer reflects a normal productivity condition and a high oxic bottom water environment with the depositional interface under the CCD. The red marl from the 35³6th layers reflects a normal productivity and a high oxic bottom water environment with the depositional interface above the CCD. The red limestone from the Tianba-B section shows a low productivity and a high oxic bottom water condition. Intensive circulation of bottom waters with higher contents of dissolved oxygen may have caused the formation of the CORBs.The Scaglia Rossa Formation from Italy reflects a normal to low productivity and a high oxic bottom water environment, with the depositional interface above the CCD. The more oxic condition was related to more intensively oxic bottom water circulation.The CORBs from the ODP1049C-12X core can be divided into two categories of"red clay a"and"red clay b". The"red clay a"deposited in the vicinity of the CCD, reflecting a normal productivity and a high oxic botton water condition. Intensive circulation of the bottom with higher content of dissolved oxygen may have resulted in the formation of the"red clay a". The"red clay b"deposited above the CCD where the productivity was very low. The low productivity might be closely associated with an oxic condition.